A typical disk drive storage system includes one or more magnetic disks which are mounted for co-rotation on a hub or spindle. A typical disk drive also includes a transducer supported by a hydrodynamic bearing which flies above each magnetic disk. The transducer and the hydrodynamic bearing are sometimes collectively referred to as a data head or a product head. A drive controller is conventionally used for controlling the disk drive based on commands received from a host system. The drive controller controls the disk drive to retrieve information from the magnetic disks and to store information on the magnetic disks. An electromechanical actuator operates within a negative feedback, closed-loop servo system to move the data head radially or linearly over the disk surface for track seek operations and holds the transducer directly above a desired track or cylinder on the disk surface for track following operations.
Typically the magnetic disks 2 also comprise servo sectors 18 which are recorded at a regular interval and interleaved with the data sectors 12, as shown in FIG. 1. A servo sector, as shown in FIG. 2, typically comprises a preamble 20 and sync mark 22 for synchronizing to the servo sector; a servo data field 24 comprising coarse position information, such as a Gray coded track address, used to determine the radial location of the head with respect to the plurality of tracks; and a plurality of servo bursts 26 recorded at precise intervals and offsets from the track centerlines which provide fine head position information. When writing or reading data, a servo controller performs a “seek” operation to position the head over a desired track; as the head traverses radially over the recording surface, the Gray coded track addresses in the servo data field 24 provide coarse position information for the head with respect to the current and target track. When the head reaches the target track, the servo controller performs a tracking operation wherein the servo bursts 26 provide fine position information used to maintain the head over the centerline of the track as the digital data is being written to or read from the recording surface.
Often, the servo information used by the servo system is prerecorded on the disk surfaces during manufacture of the disk drive module using a process sometimes referred to as servo writing. In a typical prior art process, each disk drive module is mounted to a servo writer support assembly which precisely locates the disk surfaces relative to a reference or origin. The servo writer support assembly supports a position sensor such as laser light interferometer (for detecting the position of the actuator which locates the heads that perform servo track writing), and a push pin, driven by a servo writer voice coil, which positions the actuator itself. The position sensor is electrically inserted within the disk drive's negative feedback, closed-loop servo system for providing position information to the servo system while the servo data is being written to the disk surfaces. The servo writer support assembly may also support a clock writer transducer which writes a clock pattern onto the disk surface which is used for temporally spacing the servo data about the circumference of each track. A cleanroom is required during the servo writing process as the hard drive assembly needs to be unsealed to allow the clock head, push pin and laser to access the actuator and disk. This is to prevent particle contamination during servo writing. Servo track writers and cleanroom processes are very costly.
Another technique for writing servo information uses the disk drive itself to write the servo information in situ. In situ recording means that the servo patterns are recorded on a fully assembled drive using the product head. This process is also referred to as self-servowriting. In the self-servowrite process, the product actuator is used to make the steps and the position steps are bootstrapped off the magnetic width of the head. The spacing between tracks and thus the number of tracks per inch is a function of the head width itself. Wide heads produce larger track spacings and narrow heads produce smaller track spacings. With a fixed mechanical distance from the inner diameter (ID) to the outer diameter (OD) on a disk drive, the wider track spacings produce less tracks and the narrower tracks spacings result in more tracks. One may produce fewer tracks than is required for the product format. The other may produce far more than required and make the areal density too stressful.
On most drive programs that use the self-servowrite process, a two pass process is used to obtain the correct number of tracks for the desired format. In that process, the first pass is run to get the track counts for the given disk real estate. Once that is done, the drive is again servowritten, this time using a different relative spacing to achieve the desired track counts. Use of a two pass process obviously doubles the test time and the number of servowriters required for a particular program.
What is needed is a one pass self-servowrite process capable of achieving the desired track counts on the disk drive to reduce test time and number of servowriters. Additionally, use of a self-servowrite process is more desirable as the drive does not need to be open and therefore can be run outside of the clean room, using less expensive factory space. It also saves the conventional servo track writer cost.